Triglycerides or triacylglycerols (TAG) represent a major form of energy storage in mammals. TAG's are formed by the sequential esterification of glycerol with three fatty acids of varying chain lengths and degrees of saturation. TAG synthesized in the intestine or liver are packaged into chylomicrons or very low-density lipoprotein (VLDL), respectively, and exported to peripheral tissues where they are hydrolysed to their constituent fatty acids and glycerol by lipoprotein lipase (LPL). The resultant non-esterified fatty acids (NEFA) can either be metabolised further to produce energy or reesterified and stored.
Under normal physiological conditions, the energy-dense TAG remains sequestered in various adipose depots until there is a demand for its release, whereupon, it is hydrolyzed to glycerol and free fatty acids which are then released into the blood stream. This process is tightly regulated by the opposing actions of insulin and hormones such as catecholamines which promote the deposition and mobilization of TAG stores under various physiological conditions. In the post-prandial setting, insulin acts to inhibit lipolysis, thereby, restraining the release of energy in the form of NEFA and ensuring the appropriate storage of dietary lipids in adipose depots. However, in patients with type 2 diabetes, the ability of insulin to suppress lipolysis is ameliorated and NEFA flux from adipocytes is inappropriately elevated. This, in turn, results in increased delivery of lipid to tissues such as muscle and liver. In the absence of energetic demand the TAG and other lipid metabolites, such as diacylglycerol (DAG) can accumulate and cause a loss of insulin sensitivity. Insulin resistance in muscle is characterized by reduced glucose uptake and glycogen storage, whilst in the liver, loss of insulin signaling gives rise to dysregulated glucose output and over-production of TAG-rich VLDL, a hallmark of type 2 diabetes. Elevated secretion of TAG-enriched VLDL, so called VLDL1 particles, is thought to stimulate the production of small, dense low-density lipoprotein (sdLDL), a proatherogenic subfraction of LDL that is associated with elevated risk of coronary heart disease.
Diacylglycerol acyltransferases (DGAT) catalyze the terminal step in TAG synthesis, specifically, the esterification of a fatty acid with diacylglycerol resulting in the formation of TAG. In mammals, two DGAT enzymes (DGAT1 and DGAT2) have been characterized. Although these enzymes catalyze the same enzymatic reaction their respective amino acid sequences are unrelated and they occupy distinct gene families. Mice harboring a disruption in the gene encoding DGAT1 are resistant to diet-induced obesity and have elevated energy expenditure and activity. Dgat1−/− mice exhibit dysregulated postaborpative release of chylomicrons and accumulate lipid in the enterocytes. The metabolically favorable phenotype observed in these mice is suggested to be driven by loss of DGAT1 expression in the intestine. Importantly, despite a defect in lactation in female Dgat1−/− mice, these animals retain the capacity to synthesize TAG suggesting the existence of additional DGAT enzymes. This observation and the isolation of a second DGAT from the fungus Mortierella rammaniana led to the identification and characterization of DGAT2.
DGAT2 is highly expressed in liver and adipose, and unlike DGAT1, exhibits exquisite substrate specificity for DAG. Deletion of the DGAT2 gene in rodents results in defective intrauterine growth, severe lipemia, impaired skin barrier function, and early post-natal death. Due to the lethality caused by loss of DGAT2, much of our understanding of the physiological role of DGAT2 derives from studies performed with antisense oligonucleotides (ASO) in rodent models of metabolic disease. In this setting, inhibition of hepatic DGAT2 resulted in improvements in plasma lipoprotein profile (decrease in total cholesterol and TAG) and a reduction of hepatic lipid burden which was accompanied by improved insulin sensitivity and whole-body glucose control. Although the molecular mechanisms underlying these observations are not fully elucidated, it is clear that suppression of DGAT2 results in a down-regulation of the expression of multiple genes encoding proteins involved in lipogensis, including sterol regulatory element-binding proteins 1c (SREBP1c) and stearoyl CoA-desaturase 1 (SCD1). In parallel, oxidative pathways are induced as evidenced by increased expression of genes such as carnitine palmitoyl transferase 1 (CPT1). The net result of these changes is to decrease the levels of hepatic DAG and TAG lipid which, in turn, leads to improved insulin responsiveness in the liver. Furthermore, DGAT2 inhibition suppresses hepatic VLDL TAG secretion and reduction in circulating cholesterol levels. Finally, plasma apolipoprotein B (APOB) levels were suppressed, possibly due to decreased supply of TAG for lipidation of the newly synthesized APOB protein. The beneficial effects of DGAT2 inhibition on both glycemic control and plasma cholesterol profile support that this target might be valuable in the treatment of metabolic disease.